1. Field of the Invention
This invention relates to waveguide polarizers, that is, to sections of waveguide arranged to alter the propagation modes of a wave so as to produce elliptical or circular polarization.
2. Description of the Prior Art
Many arrangements have been proposed for altering the polarization of waves as they are propagated through sections of waveguides. The prior art provides numerous examples of waveguide polarizers in which a waveguide that can support two spacially orthogonal independent waveguide modes is provided with discrete inductive or capacitive loading, dielectric loading or dimensional perturbations to introduce differential phase shift between the two orthogonal components. Such a polarizer is frequently used in circularly polarized antenna systems in which the waveguide polarizer section is interposed between a horn radiator and a waveguide section that supports a linear polarized wave.
U.S. Pat. No. 2,607,849 to Purcell et al. describes a waveguide for producing, from plane-polarized components, circular polarization of various degrees of elliptical polarization by means of slabs or plates of solid dielectric material extending lengthwise in the waveguide. The incident wave transmitted to the waveguide is polarized so that its electric vector is at an oblique angle with respect to the surface of a dielectric plate extending across and longitudinally within the waveguide. The component waves having electric vectors oriented in a plane parallel with the surfaces of the dielectric plate will be propagated at a velocity different from those having electric vectors oriented perpendicularly to the surfaces of the plate. This difference in velocity arises because the plate has a relatively smaller effect upon an electric field directed perpendicularly to the surfaces of the plate whereas it has relatively large effect upon an electric field in which the electric vector lies in a plane parallel with the surfaces of the plate. The length of the plate is selected to provide the desired ellipicity of polarization.
A somewhat similar arrangement is shown in U.S. Pat. No. 2,546,840 to Tyrrell that makes use of one or more metal fins attached within the waveguide so as to possess both radial and longitudinal extent. The effect of the fins on wave transmission depends upon their orientation with respect to the polarization of the waves. Such a fin alters the phase velocity and critical cut-off frequencies for polarization or orientation of a field parallel thereto, but has no effect on corresponding perpendicular polarizations. The fin is dimensioned and shaped to provide the desired degree of phase shift. The phase shift section is matched to the main waveguide over a broader band of frequencies by the use of tapered or reduced cross sections formed on the fin.
U.S. Pat. No. 2,599,753 to Fox shows a fin formed by dielectric material extending partially or completely across the waveguide. Broader band operation is said to be achieved by capacitance reactance screws extending into the waveguide in the region of the fin and so oriented and adjusted as to provide a compensation and neutralizing action. The end portions of the fin are either provided with a V-shaped notch or a tapered pointed section to minimize discontinuities.
U.S. Pat. No. 2,858,512 to Barnett shows a phase shifter making use of fins of dielectric material positioned in a circular section of waveguide that, by means of flange connections, can be rotated relative to the adjacent waveguide sections for mechanical adjustment of the phase shift.
U.S. Pat. No. 2,933,731 to Foster describes the use of either a dielectric strip, metal fins or a metal plug in much the same manner as the earlier prior art to achieve circular polarization. Also disclosed is the use of a section of waveguide elliptical in cross section to replace the use of either the dielectric strip, the fins or the plug. The elliptical cross section may be obtained by distortion of a section of circular waveguide.
U.S. Pat. No. 3,031,661 to Moeller et al. shows an arrangement interposed between a square waveguide and a radiating horn to provide circular polarization. A slab of dielectric material is positioned in a circular section of waveguide that is mechanically rotatable to alter the orientation of the dielectric slab. The radiating horn is provided with a series of discrete inwardly extending fins on each of the four sides that are said to produce horn patterns independent of polarization.
U.S. Pat. No. 4,141,013 to Crail et al. discloses various arrangements of conductive fins (irises) extending from the waveguide walls. Also described is a horn having spaced fins (irises) extending from opposite corners of the horn. Each pair of conductive fins imparts a rotation or circular polarization in a linear wave propogating past each pair.